This invention relates generally to electrochemical gas sensors, and, more particularly, to an improved sensing electrode structure for electrochemical gas sensors. Gas sensors of this type are used to measure the partial pressure of a gas in a mixture of gases. For example, by appropriate selection of components, a gas sensor can indicate the concentration of oxygen in air. Other sensors indicate the concentrations of carbon monoxide, sulfur dioxide, oxides of nitrogen, hydrogen sulfide, and various other gases.
Basically, an electrochemical gas sensor of the type with which the invention is concerned includes a container in which there are disposed an electrolyte, a sensing electrode, and a counter electrode. When a gas to be sensed is introduced into the electrolyte adjacent to the sensing electrode, ions are formed and act as current carriers. A measureable current can then be detected in an external circuit connected to the electrodes.
There are two basic categories of sensors of this general type. One is the galvanic type, in which the counter electrode and electrolyte are selected to provide a measureable current without any external voltage source being necessary. For example, the use of lead or cadmium as a counter electrode, in an alkaline electrolyte, provides a galvanic sensor for the measurement of oxygen concentration. Also, the use of lead dioxide or manganese dioxide as the counter electrode, in an acid electrolyte, provides a galvanic sensor for the measurement of concentrations of carbon monoxide, hydrogen sulfide, and sulfur dioxide. The other type of sensor is referred to as the polarographic type, in which the counter electrode is of such a nature as to require the use of an external voltage source to make the sensor operate. The magnitude of the required external voltage will depend on such factors as the nature of the counter electrode, the acidity (pH) of the electrolyte, and the gas to be measured. The present invention is not limited to either the galvanic or the polarographic type of sensor.
An important consideration in one application of gas sensors is that the readings obtained should be proportional to the partial pressure of the gas to be measured. For example, in monitoring the partial pressure of oxygen in air for health reasons, the reading should reflect changes in total air pressure, even though the oxygen concentration may not have changed. In other words, the sensor should be indicative of the total amount of oxygen available, which is proportional to partial pressure, rather than indicative of the concentration of oxygen by weight or volume.
A typical oxygen-sensing device of the prior art is described in U.S. Pat. No. 3,429,796, issued in the name of Jay M. Lauer. The sensing electrode in the Lauer device is a metallic mesh on which has been deposited a layer of silver or gold. The gas to be measured is introduced through a membrane of solid Teflon (DuPont) having a thickness of 0.001 to 0.002 inch. Typically, sensors of this type have sensing electrodes of about one-inch diameter, and generate currents in the range of 200 to 1,000 microamperes. The current is a measure of the useful life of the sensor. A lower operating current will result in a longer useful life. Sensors of this type are also very temperature dependent, since the gas has to permeate through a non-porous membrane. Temperature variations may result in readings differing by as much as 5-10% per degree C. In addition, the thin membrane renders the device very fragile and susceptible to electrolyte leaks.
Another sensor is described in U.S. Pat. No. 4,132,616, issued in the names of Anthony D. S. Tantram et al. In the Tantram device, the sensing electrode is approximately 0.5 inch in diameter and is made of nickel gauze pressed onto a porous Teflon tape of about 0.008 inch thickness. Between the Teflon and the gas sample is a plug with a capillary to reduce the flow of gas to the sensing electrode. However, the operating current level is still at approximately the 1,000 microampere level, thus significantly limiting the useful life of the device. In addition, the capillary arrangement renders the sensor insensitive to changes in partial pressure, and is therefore not suitable for human safety applications.
It will be appreciated from the foregoing that there is still a significant need in the gas sensing field for an electrochemical gas sensor that overcomes the problems of the prior art. In particular, what is needed is a sensor with a relatively low working current, to provide a long useful life, insensitivity to temperature variations, and the ability to respond relatively quickly to changes in partial pressure of the sensed gas. The present invention satisfies these requirements.